The present invention relates to vehicles either non powered such as trailers or powered such as pick up trucks, trucks and vans, having a vertically adjustable bed.
Standard vehicles such as vans and trucks have been used for the transportation of cargo. However, because of the height of the tray or bed above the ground, loading, unloading and positioning of cargo can be difficult.
In practice, devices such as fork lifts, lifting tailgates and ramps are used. These however have their disadvantages such as costs and availability at any particular site.
Attempts to overcome the above disadvantages have been made utilizing either pivoting trays or adjustable trays in which parallelogram linkages are employed.
With the pivoting type trays such as is shown in U.S. Pat. No. 5,288,197, the back end of the tray is pivoted to the ground, forming a ramp up which the load is driven, in the case of vehicles, or pushed. This method of loading is not always convenient due to the problems of the loading onto an inclined ramp.
Adjustable trays have been proposed as shown in Australian Patent 57 3414 and Australian Patent Application No. 70142/91. In both of these specifications, parallelogram linkages in the form of "dog lcg" or "boomerang" linkages are used, with the power means or rams acting on the linkages normal to the displacement of the tray. Further in both of these specifications the tray swings to a position rearway of the back of the vehicle, which could present problems in use. In U.S. Pat. No. 4,673,328 again a parallelogram type linkage is used having four pivotable tracks.
The construction shown in Australian Patent Application No. 70142/91 utilizes a complicated chassis construction with a torque box, rollover bar and support bars required to give stability to the yoke shaped chassis.